1. Field of the Invention
The present invention relates to a hologram recording and reproducing apparatus that records and reproduces information on and from a holographic recording medium.
2. Description of the Related Art
There is known a hologram recording technique which records information to be recorded on a holographic recording medium (hereinafter simply referred to as a “recording medium”) as interference fringes. One method for this technique uses the information to be recorded to spatially modulate a light from a light source to generate an object light. The apparatus irradiates the recording medium with an object light and a reference light. The object light and the reference light generate interference fringes on the recording medium, and the interference fringes are recorded in a recording layer of the recording medium. On the other hand, for reproduction, the interference fringes recorded on the recording medium are irradiated only with the reference light. A two-dimensional sensor detects a detected light from the recording medium to reproduce the recording information.
Similarly to the recording of information on an optical disc or the like, hologram recording focuses the object light and the reference light on the recording medium. Accordingly, to correctly record and reproduce information, it is necessary to perform what is called focus servo control for controlling the position of an objective lens with respect to the recording medium. Japanese Patent Application Laid-Open under No. 2002-123949 describes an example of focus servo control performed in a hologram recording and reproducing apparatus to generate a focus error signal and thereby control driving of the objective lens. For precise focus servo, it is important to appropriately set a target value for focus servo. However, Japanese Patent Application Laid-Open under No. 2002-123949 does not disclose any method for determining a target value for focus servo.
Other documents, for example, Japanese Patent Applications Laid-Open under Nos. 2002-63733 and 2000-122012 also describe hologram recording.